Some authors have been able to assess degradation kinetics by correlating oil residence times in the subsoil with the biodegradation level (Larter et al, 2003 Organic Geochemistry 4, 6001-613, Behar et al, 2006, Organic Geochemistry 37, 1042-1051 et de Barros Penteado et al, 2007, Organic Geochemistry 38, 1197-1211). It essentially consists of a descriptive work that aims to record the biodegradation and its consequences, and to determine specific biodegradation rates for some basins. In particular, Larter's work allows to predict a biodegradation rate without taking account of the oil composition, this rate being mainly applied to saturated hydrocarbons. Besides, the mainspring of biodegradation is the diffusion of hydrocarbons in the water/oil contact zone, insofar as this contact zone provides the source of electron acceptors.
A method of predictively assessing the oil biodegradation level in a basin is known from patent EP-1,436,412. This methodology is based on a statistical analysis of the number of bacteria present in the subsoil as a function of depth. Besides, the result of this approach is not compositional.
In general terms, the known methods aim either to describe the biodegradation and not to predict it, or to predict the biodegradation by taking account of the water/oil contact plane to control the biodegradation velocities. These types of method do not take account of factors limiting the activity of micro-organisms: the electron acceptors. Thus, these methods are not really predictive and they require calibration with well data.
The object of the invention thus relates to an alternative method of assessing the compositional evolution of hydrocarbons trapped in a porous medium as a result of biodegradation.